Separation methods for water treatment include a heating or phase-change method, a filtration membrane method, and the like. According to the filtration membrane method, it is possible to obtain the water of quality as desired by controlling the size of the fine pores of the filtration membrane, which advantageously improves the reliability of process. Furthermore, since the filtration membrane method does not require a heating process, it can be advantageously used for water treatment using microorganisms that could be adversely affected by heat.
Among the filtration membrane methods is a method using a hollow fiber membrane module in which a bundle of hollow fiber membranes are disposed. Conventionally, the hollow fiber membrane module is widely used in the field of precision filtration such as preparation of sterile water, drinking water, ultrapure water or the like. Recently, however, the application field of the hollow fiber membrane module is extending to sewage/waste water disposal, separation of solids in sewage disposal tank, removal of suspended solids (SS) in industrial waste water, filtration of stream water, filtration of industrial water, filtration of pool water and the like.
Generally, a filtration apparatus using hollow fiber membranes has a bundle of hollow fiber membranes of a certain length and is classified into a submerged-type and a pressurized-type depending on the operation manner thereof.
In case of a pressurized-type filtration apparatus, as a fluid to be processed is pressurized, only a pure fluid is allowed to permeate the hollow fiber membrane and come into the hollow thereof while the contaminants such as impurities or sludge remains outside the membrane. Although requiring additional facilities for a fluid circulation, the pressurized-type filtration apparatus is advantageous in that it can produce more filtrate for an unit time than a submerged-type filtration apparatus.
On the other hand, in case of a submerged-type filtration apparatus, a hollow fiber membrane module is submerged in a bath containing a fluid to be processed. As a negative pressure is applied inside the hollow fiber membrane, only pure fluid is allowed to permeate the membrane and come into the hollow thereof while the contaminants such as impurities or sludge remains outside the membrane. Although producing less filtrate for an unit time than a pressurized-type filtration apparatus, the submerged-type filtration apparatus is advantageous in that it can decrease the costs for setting up the facilities and for operating thereof since it does not require any facility for the fluid circulation.
Both pressurized-type and submerged type filtration apparatuses may be classified into a double-ends collecting type which collects the filtrate permeating the hollow fiber membrane and coming into the hollow thereof through the both ends thereof and a single-end collecting type which collects the filtrate only through one end of the hollow fiber membrane.
Any of the pressurized-type and submerged type filtration apparatuses requires a filtrate collecting space for collecting the filtrate which has permeated the hollow fiber membrane and come into the hollow thereof, and a means for spatially separating the filtrate collecting space from the fluid to be treated. The means for spatially separating the filtrate collecting space from the fluid to be treated is often referred as a fixing layer since it also performs a function for fixing a bundle of hollow fiber membranes in a header. The bundle of hollow fiber membranes are potted in the fixing layer.
As the filtration process is performed, the impurities existing in the fluid to be treated attaches to the surface of the filtration membrane thereby causing the fouling problem, i.e., the contamination of the filtration membrane. The fouling of the membrane reduces the filtration efficiency of the filtration membrane. Thus, it is necessary to clean the filtration membrane during the filtration process. Generally, an aeration cleaning is carried out to clean the filtration membrane during the filtration process. According to the aeration cleaning method, an aeration tube disposed below the filtration membrane module strongly eject the bubbles which impact with the filtration membrane thereby removing the impurities attached to the surface thereof.
The aeration cleaning, however, causes the vibration of the filtration membrane, weakens the adhesion between the header and the fixing layer in which the filtration membrane is potted, and finally causes the separation of the fixing layer from the header.